A time-dependent diffusive model for the simulation of chloride penetration in concrete considering the effect of natural salt freeze-thaw cycles

In marine cold regions, reinforced concrete (RC) structures are affected by the coupling action of chloride erosion and freeze-thaw (FT) cycles. The coupling effects will lead to the concrete deterioration, accelerate the chloride diffusion process, and significantly reduce the service life of RC structures. In this investigation, a chloride diffusive model considering the time-varying effects of FT damage and temperature was proposed to quantify the diffusion of chloride in concrete under the action of natural salt freeze-thaw cycles (SFTCs) or under the action of alternating environment, in which the salt FT cycling environment and the pure chloride diffusion environment appear alternately. In addition, a natural salt FT cycling experimental method was put forward, which can simulate the actual salt FT cycling process of concrete in tidal zone. Chloride content and the dynamic elastic modulus (DEM) of specimen were determined after corresponding exposure duration, i.e., 30 d, 60 d, 80 d, 100 d and 120 d. Meanwhile, during the exposure time, the temperature at different positions (air temperature, water temperature, central temperature of the specimen in atmospheric area, tidal zone, and underwater area) was monitored continuously. The results show that the maximum number of FT cycles in this experiment is 36, and the average FT minimum temperature is-4.3 C. The cycle of FT action is 24 h, and the ratio of freezing time and thawing time is about 5:1. After the initiation of FT action, the DEM decreases linearly with exposure time, with maximum loss of about 3% in one winter. Compared with the previously reported rapid FT cycling test data, the loss of DEM caused by one rapid FT cycle is about 9.3 times of that caused by one natural salt FT cycle. Besides, the mass loss of concrete under natural SFTCs for one winter in Tianjin area is about 0.3%. And chloride profiles in concrete deepened with exposure duration. Subsequently, the proposed model was used to simulate the diffusion of chloride in concrete under the experimental conditions. The chloride contents simulated by the proposed model agree well with the test results, showing that this model is applicable to predict the diffusion of chloride in concrete exposed to the action of alternating environment.